In its simplest form, the corrugating line includes a so-called single facer and a so-called double backer which are spaced apart from one another under a horizontal bridge. A corrugating medium, such as paper web, is drawn from a supply roll located under the bridge into the single facer. A second paper web, the so-called liner, drawn from a second supply roll located under the bridge is also fed into the single facer. The single facer subjects the incoming corrugating medium to steam and passes it between a pair of corrugating rollers which cause the medium to assume its familiar fluted or rippled configuration. Thence, the fluted web is passed through a gluing station which applies glue to the tips of the flutes at one side of the web. The liner material is, in turn, fed over a series of rollers and brought into contact with the glue-bearing side of the fluted web. Thus the two webs are laminated so that so-called single-face web (which is longitudinally flexible and laterally rigid) emerges from the single facer.
An inclined, double-belt sandwich conveyor is located above, and is driven by, the single facer. This conveyor is run slightly overspeed to impart tension to the single-face web. The conveyor carries the single-face paper upward and away from the single facer and drops it onto a horizontal belt conveyor located on the bridge and which extends all the way along the bridge to the double backer. The bridge conveyor is operated at a slower speed than the inclined conveyor causing the single-face web to fanfold in loops as it falls onto the conveyor belt. This permits the bridge conveyor to store a relatively large quantity of single-face web. The belt conveyor thus forms a buffer or accumulator for supplying the needs of the double backer when the single facer is slowed down during adjustments or paper changes. Likewise, if the double backer is slowed down or stopped, the single facer can continue to run at normal speed building up the supply of web on the conveyor. The storage of the single face material on the belt conveyor is also important because it provides extra time for drying and bonding of the single face material to insure a more permanent adhesion of the fluted web to the liner web.
Upon leaving the belt conveyor, the single-face web is drawn down into a glue station just ahead of the double backer there glue is applied to the tips of the exposed flutes on the single-face material. Also, web which will constitute the bottom liner of the corrugated board is drawn from a supply roll under the bridge through a preheater into the double backer. In the double backer, the prepared single-face material and the linear material are brought together at a nip where they adhere, thereby forming the completed corrugated board. By the time the board leaves the double backer, the three webs have dried and the glue has set so that the laminated board can be cut into sheets and prepared for shipping or storage.
The usual corrugating line also includes various stations for preconditioning and preheating the webs prior to their entrance into the single facer and double backer.
A major problem with prior corrugating lines of the type just described is that there are variations in the degree of drying of the glue on the two layers of liner web. More particularly, the moisture content of the webs and the degree of drying of the glue on the single-face web depends upon the length of time that the paper spends on the belt conveyor on its way to the double backer. Since the amount of material on the bridge is not known with any degree of certainty, an excessive amount is usually stored there so that there is a considerable delay before the material reaches the double backer. Resultantly, the single face material must be preconditioned by steaming it before it enters that unit.
Also the elongations of the webs comprising the board vary due to changes in humidity and temperature and upon the fact that the untensioned single face material on the conveyor is left to dry in accordion folds. Also, of course, the stresses in the single face material may vary along its length and width depending upon its lie on the belt conveyor. On the other hand, some shrinkage of the single-face material may occur as it proceeds along the conveyor. All of these factors cause warpage in the finished corrugated board as well as variations and irregularities in the flutes or corrugations along and across the board.
A further disadvantage of the prior corrugators stems from the fact that a series of finger crescents are positioned along the length of the corrugator roll adjacent the gluing station to control the amount of glue that is applied to the flutes of the corrugating medium. Due to the construction and operation of the single facer, the edges of the single face web in that unit must lie between adjacent finger crescents to avoid damage to the web. This means that the web leaving the single-facer may not be aligned with the machine center-line of the double backer.
To compensate for this misalignment, a pair of side guards or shoes are mounted adjacent each edge of the web at the entrance end of the belt conveyor positioned so as to establish the desired path of web travel. If the web is out of alignment, one shoe or the other pushes against the web edge so that the web more or less tracks along the desired alignment as it approaches the double backer. These shoes inevitably crush the edges of the single face web so that the corrugated board emerging from the double backer may be defective along one or both of its edges. Furthermore, these shoes must be adjusted during a width change and, if adjusted too soon or too late, loss of steering or crushed web can result. Actually, in many cases the damage is sufficiently extensive that the corrugated board has to be edge-trimmed in order to remove the defective material. This, of course, creates wastage and increases costs. Paper webs wider than the product are used to allor for this trim.
A further disadvantage of the prior apparatus of this general type is that, due to the fanfold mode of storing single-face material on the belt conveyor, one cannot know with any reasonable degree of certainty how much web is stored on the conveyor at any given time. Therefore, an operator has to frequently change the speed of the single facer in order to avoid feeding too little or storing too much single-face web on the bridge.
Lack of knowledge of the precise amount of material in buffer storage also makes it difficult to use up the material on the conveyor belt to avoid wastage when changing over from one web to another, for example, when changing flute size or board width.
For all of the aforesaid reasons, conventional corrugator lines are not as productive and efficient as they might be. Also they do not produce corrugated board of as high a quality as might be desired.